1. Field of the Invention
The present invention relates generally to weighing apparatus of the electromagnetic-load-compensation type, and particularly to shielding means for use therewith to render its weight measurements immune to magnetic interference.
2. Description of the Prior Art
Weighing apparatus of the electromagnetic-load-compensation type are well known. Shielding means for protecting such apparatus from stray magnetic flux produced internally by the permanent magnet system within such apparatus also are known. For example, U.S. Pat. No. 4,090,575 discloses weighing apparatus of the foregoing type having a magnetically conductive shield for protecting its compensation coil from such internally generated flux. Also, U.S. Pat. No. 4,545,448 discloses weighing apparatus of the same type having its compensation coil enclosed by magnetically conductive parts of the permanent magnet system itself, to eliminate stray flux which otherwise might result from that system, and thereby preclude any adverse interaction between such flux and nearby ferromagnetic objects.
While such known shielding means may have sufficed for the purpose of protecting the load-compensation coil in such apparatus from adverse effects of internally created magnetic flux, there has remained a persistent need for an effective way to shield, not only the compensation coil, but all parts of such apparatus from interference that could be caused by an external source of magnetic flux. A common example of an external source is a stirring magnet used with a mass to be weighed in a container therefor on the apparatus weighing pan. It has been found that the magnetic field created by a stirring magnet interferes adversely with the weight-measuring function of such apparatus, and thereby causes significant weighing errors. Although magnetic stirrers have been widely used for some time, and the need to eliminate such errors has been well recognized, a practical and efficient way to meet that need, without adversely affecting the apparatus itself or its intended operation, has not been realized until the advent of this invention.